The principal paradigm of telescopic rifle sight production is production on the lathe. Although this facilitates the centering of the scope components, the resultant scope housing must be round in transverse section. There may be some instances in which a scope that is round in transverse section is not optimal.
One difficulty in assembling a scope is the need to critically adjust the distance between the lenses of the scope, so that the reticle is in focus at every power of magnification. In a standard tube housing scope it may be challenging to make some of these adjustments, because of a lack of space. Also, in a standard tube housing scope there is a shortage of space for some of the inner workings, forcing in some cases the use of thinner walled materials than is desirable.